Printed wiring boards (PWBs) are an integral part of electronic equipment and there have been continuing efforts to increase the interconnection density and electrical efficiency of PWBs and thus decrease their size and cost. The technology relating to PWBs goes back a number of years and, in general, a printed circuit is patterned on a copper foil which is located on the surface of a dielectric material such as a plastic substrate. These boards vary in design and may have a copper foil on each surface of the plastic substrate, usually epoxy, (termed 2-sided boards) or they can be multi-layer boards which have a plurality of interleaved parallel planar copper foils and epoxy layers. In both types, through-holes are drilled in the board and metal plated to facilitate connection between the copper foil circuits.
Unfortunately, the copper plated through holes used in these conventional structures are susceptible to barrel cracking during qualification testing, e.g., thermal cycling. The barrel cracking, as those skilled in the art are aware, negatively affects the electrical characteristics of the PWB. The industry has contemplated drilling a small enough through hole in the PWB that the copper plating would fill and thereby prevent the barrel cracking. Unfortunately, the through holes have typically high aspect ratios, and the filling of the holes with copper using the plating technique would undeniably block one end of the hole, thus trapping unwanted matter therein. Again, the electrical characteristics of the PWB would be compromised. Additionally, during plating of the through holes copper is also deposited on the flat external surfaces of the PWB, resulting in thicker copper layers. These thicker copper layers tend to impede subtractive fine line patterning.
Accordingly, what is needed in the art is an electrical connection between PWB layers that does not experience the problems or drawbacks experienced or introduced by the prior art electrical connections.